The role of chromosome ends in initiation of synapsis has been investigated through electron microscopic examination of pachytene nuclei of three mouse translocations involving chromosomes 7 and the X. Two of these translocations were reciprocal with ends available for initiation. These translocations exhibited a high rate of quadrivalent formation and homologous pairing. The third (Cattanach's) is an interstitial translocation in which a section from the middle of 7 was translocated to the distal third of the X. Consequently there is no end available for initiation of synapsis of this region. Bivalent 7 pairs with the homologous region at the X only 24% of the time, indicating a greatly reduced efficiency of recognition and initiation for interstial segments. The organization of prophage chromosomes within nuclei is being investigated by taking a series of light micrographs of serial sections. This information is being transferred to a computer and the 3-D structure of the nuclei are being graphically reconstructed. This image will then be compared with a 3-D model of a hypothetical nucleus, whose structure is assumed on the basis of certain inferred "rules" of chromosome behavior.